The in situ oxidation of the petroleum resin under soft condition was investigated. The oxidation was carried out by using hydrogen peroxide and acetic acid as the oxidants, and sulphuric acid as a catalyst for in situ formation of the peracetic acid. The infrared spectroscopy analysis shows the changes in the molecular structure: the amount of the epoxy, hydroxyl and carbonyl groups increases and the number of unsaturated bonds decreases. The new method for the determination of the reaction rate constant was presented. For the determination of the reaction rate constants the relative intensity of absorption band in the area of epoxy and carboxyl groups vibration was used. Obtained data are in a very good correlation with the reaction rate constants obtained with the epoxy and acid number valuation. That allows to conclude the validity of using presented algorithm. Kinetic calculation indicates the occurrence of the oxidation reaction by the first order. According to average reaction rate constant the process has a high selectivity for epoxy groups.